Interpretive Summary: Rush skeletonweed is an important invasive plant in western North America, and considered the worst invasive plant in Idaho and the most problematic weed of wheat growing regions in Australia. Some of the rush skeletonweed varieties in the USA and Australia are resistant to biological control fungi used to control them. We used genetic data to identify how many genotypes of rush skeletonweed are in the invaded countries, and then we compared those to collections from the source range (Eurasia) so that we could find the origins of invasive genotypes. We were able to find exact matches for five of the 13 invasive genotypes, and highly similar matches for three other genotypes . This will allow biological control researchers to go to exact locations in Eurasia to search for additional fungi that should attack these invasive genotypes. Also, we determined where the USA and Australia genotypes are distributed so that future releases of fungi will not be wasted on areas where plants genotypes are known to be resistant to a specific strain of fungus.

Technical Abstract:
Assessing the propagule pressure and geographic origins of invasive populations using molecular markers provides insights into the invasion process. Rush skeletonweed (Chondrilla juncea) is an apomictic perennial plant that is invasive in Australia, Argentina, Canada and the USA. Invasive biotypes from various invaded areas have been described using morphology, phenology, allozyme diversity, and resistance to control agents. However, no study has comprehensively compared native and invasive populations from the three invaded regions using a highly polymorphic molecular marker system. In this study, we analyzed 1056 native range plants from Eurasia and 1156 plants from three invaded regions (North America, Australia and Argentina) using Amplified Fragment Length Polymorphic (AFLP) techniques. We found 682 unique AFLP genotypes in the native range, but only 13 in the three invaded regions. Each invaded region contained distinct AFLP genotypes, suggesting independent introduction events, probably with different geographic origins. This comparison of native and invasive populations indicates that relatively low propagule pressure is associated with the introduction of rush skeletonweed around the globe. Our results increased the number of known genotypes in North America from three to seven, but we failed to find additional genotypes in Australia and Argentina. AMOVA analysis indicated that genetic diversity in the native range is partitioned equally within and among populations. In Argentina, most of the genetic diversity is found within populations, whereas in North America and Australia diversity is mostly found among populations. There was a significant correlation between geographical and genetic distance in the native range, and we found exact AFLP genotype matches between the native and invasive ranges for five of the 13 invasive genotypes. We found high genetic similarity (> 0.95) for three other genotypes. The results of this analysis shed new light onto the invasion of rush skeletonweed around the globe, and they also hold significance for the management of this destructive plant, especially future biological control efforts.